1. Field of the Invention
The present invention relates to a system for relieving a digital transmission link having N bidirectional working lines and P bidirectional spare lines where N&gt;P. This link is connected between two end stations of a digital transmission system. Each end station comprises a bidirectional switching unit controlled by a monitor which exchanges order signals with the other end station through the link.
2. Description of the Prior Art
The article by A. Chomette, J. Guiomar and M. Robinet entitled "Securisation et Supervision d'un reseau numerique urbain" (relieving and monitoring an urban digital network), published in the French review "l'Echo des Recherches", January 1979, pages 12 to 19, describes a method for relieving a working line in a digital transmission link. According to this article, the prime function of a switching unit consists of switching the ends of a working line over to a pre-selected spare line, as soon as a transmission fault in the working line is detected. This precludes virtually any loss of the data transmitted by the failed line, i.e. a break in the communications in progress carried by the working line whilst the repair thereof is going on.
With this in mind, the emitting end of the switching unit in an end station that is intended to emit data to the other station via the transmission link comprises, for each working transmission line, first switching means that are associated with each spare line. After an alarm indicating a transmission fault in the said working line has been detected, the first switching means allows the emitting end of the faulty working line to be connected to the spare line, in order to transmit data related to the faulty line. These switching means are, when there is no alarm, connected solely to the spare line. When this spare line is selected, the switching means enables the monitor to emit first order signals to the other end station, prior to any first switch-over. The receiving end of the switching unit in an end station which is intended to receive data from the other end station via the transmission link, comprises, for each working transmission line, second switching means connected to each spare line. After an alarm related to the failed line has been detected, the second switching means makes it possible to connect the spare line to the receiving end of the failed line for receiving data related to the failed line. The second switching means are, in the absence of an alarm, connected solely to the spare line. The second switching means enables, via this selected spare line, the monitor to receive the first order signals coming from the switching unit of the other end station. Thereafter, the second switching means enables the monitor downstream of the faulty line to receive the data related to the faulty line whilst being repaired. Finally, the second switching means enables the monitor to receive second order signals after the alarm due to the line repair has ceased. The second and last exchanges of order signals are also performed along the selected spare line and, as a result, require a momentary break in the communications related to the line. Following this second order exchange, a second switch-over re-establishes normal transmission along the repaired line.
In the straightforward case of the number of spare lines being equal to unity (P=1), a relief method of this type does not provide for a line being available for transmitting orders when a working line has been relieved. Indeed, two cases may arise in which it is important to be in a position to exchange orders between the switching units of the two end stations. The first case is that of a second working line in the same link becoming faulty. The second case is that of the faulty working line, with communications routed via the spare line, being restored to working order.
In the first case, if the second faulty working line has priority, there is no drawback in breaking the spare line traffic for conveying order signals and in assigning this spare line to the relief of the priority failed working line; should, however, the second faulty line not have priority over the first, then said second working line is not relieved and its traffic is interrupted.
In the second case, in relation to repairing the faulty line, second order signals must be exchanged before ordering the switch-over of traffic from the spare line to the repaired working line. According to the preceding article, this second exchange of order signals can also come about by momentarily breaking the spare line traffic, which may be very annoying, in particular should the relieved line be a priority line or a line conveying data, for example, between a terminal and a computor where the transmission must not be broken.
In the article by James W. Wuertele published in the National Telecommunications Conference review, 1973, Conference Record Vol. 2, New York (U.S.), is described a relieving method which obviates the drawbacks of the method in the article by A. Chomette et al. The second exchange of order signals does not occur via the pre-selected spare line, but via the repaired line whilst the data related to this repaired line continues to be transmitted along the pre-selected spare line. As a result, these conditions do not break the traffic related to the repaired working line.
However, according to the second preceding article, the receiving end of an end station switching unit is connected to the working line throughout the repair time, just before the second switch-over, in order to receive test signals via the latter. Thus, just before the second switch-over, i.e. before the second exchange of orders, the faulty working line is connected to the order emitting and receiving circuits in the end stations. If a second working line were to fail before the first one has been repaired and the second line were to have priority over the first, no exchange of orders could occur between the stations until the first working line had been repaired.